1. Field of the Invention
The present invention relates to a laser light control device for an image forming apparatus, and an image forming apparatus.
2. Description of the Related Art
The optical power characteristics of a semiconductor laser are sensitive to changes in ambient temperature. Therefore, the optical power of the semiconductor laser varies largely due to changes in ambient temperature or self-heat generation for example, even during constant-current drive. To cope with this temperature dependence of the optical power, a photodiode (PD) called a monitor diode is ordinarily incorporated in a package for a semiconductor laser. This causes the optical power from back light from the semiconductor laser to be monitored, thereby providing a control of the drive current to constantly maintain the optical power. A drive circuit having such a feedback mechanism to control the light quantity of a semiconductor laser is referred to as an auto power control (APC) circuit (automatic optical power control circuit).
In a drive circuit which turns on/off a semiconductor laser according to image data in a printer with a laser light control device, or the like, the auto power control (APC) is ordinarily performed by lighting a laser for a certain time period outside an image area, because the laser lighting time is irregular.
An arrangement using in an APC circuit for laser scanning device a photodiode-integrated-type laser diode having a photodiode integrally incorporated in a package has advantages mentioned below. That is, it is capable of reducing the disposition space, eliminates the need for independent wiring and is advantageous in simplifying the construction of the laser scanning device and reducing the cost in comparison with an arrangement in which the emission output from a semiconductor laser is monitored with a discrete photodiode arranged separately from the laser diode.
With the above-described arrangement, however, laser light quantity control of high accuracy has not been realized even by performing APC, because the quantity of back light from the semiconductor laser received by the photodiode in the package also varies slightly with changes in the temperature of the semiconductor laser.
A system is known in which forward light emitted from a package of a semiconductor laser is used for APC (see, for example, Japanese Laid-Open Patent Publication (Kokai) No. H09-321377).
In the above-described arrangement, however, there is a need to respectively provide a photodiode for detecting the main scanning position of laser light (image writing start reference signal: BD signal) and a photodiode for detecting the quantity of light.
It is preferable to use one photodiode as a photodiode for detecting the main scanning position of laser light (image writing start reference signal: BD signal) and as a photodiode for detecting the quantity of light. However, the region in which the position of the photodiode in the laser light main scanning direction can be set is so restricted that the area occupied by the light receiving surface of the disposed photodiode is small, and the time at which the quantity of laser light is detected is the time at which laser light passes the photodiode during scanning along one line. Therefore, the time period during which the quantity of laser light is detected with the photodiode is extremely short. With such an arrangement, the quantity of laser light cannot be accurately detected if the response of the photodiode is low or the time period required for convergence of APC control is long. In such a case, it is difficult to perform feedback control with stability.